Thursday, September 24, 2009

OSPF States and related troubleshooting

  • Neighbor States

    state = down

    state = init -> multicast reachability issue

    state = 2-way ->
    2WAY/DROTHER is ok between non DR/BDRs

                For two-way communication to be established with a neighbor, a router also must see its own Router ID in the Neighbor field of the neighbor's HELLO packets.

    state = exstart -> MTU mismatch ; The router with the higher router ID becomes the master and starts the exchange, and as such, is the only router that can increment the sequence number. Note that one would logically conclude that the DR/BDR with the highest router ID will become the master during this process of master-slave relation. Remember that the DR/BDR election might be purely by virtue of a higher priority configured on the router instead of highest router ID.
    Thus, it is possible that a DR plays the role of slave.
    And also note that master/slave election is on a per-neighbor basis

    state = exchange

    state = loading -> uncommon, open TAC

    state = Full

    fully adjacent with routers with which they have successfully completed the database synchronization process.

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